1. Field of the Invention
The present invention relates generally to a supercontinuum generator for producing a multiple optical channel source, and more particularly, to a supercontinuum (SC) generator to provide a multiple optical channel source for a wave division multiplexing (WDM) communication system.
2. Description of the Related Art
SC generation refers to the generation of intense ultra-fast broadband “white light” pulses spanning from the ultraviolet (UV) to the near-infrared (near-IR) wavelength regions. SC generation is one of the key requirements necessary for developing ultra-fast laser and nonlinear optics technologies. One method of generating an SC includes inputting a high-power light pulse into a non-linear optical medium wherein the light pulse undergoes a broadening and emerges as an SC. This broadening is due to non-linear interactions between the light and the medium.
When applied to communication systems, SC wavelength-sliced sources have many advantages, chief of which are being background-noise free and having a high-bit rate capacity. Additionally, SC derived sources can reduce the hardware complexity of optical communication systems by replacing many light sources with a single SC source, which would result in commensurate savings in hardware design and testing, and reduce system maintenance and cost.
For example, in conventional wave division multiplexing (WDM) communication systems, a large number of optical sources such as laser diodes are used as a multiple channel source. Additionally, each laser diode must be independently controlled by a controller. This increases the complexity and cost of the WDM communication system and decreases the system's reliability. Moreover, it is difficult to construct existing WDM sources having more than 50 channels. Additionally, it is difficult to provide uniform channel spacing using the existing WDM sources.
SC's based on self phase modulation (SPM) and cross phase modulation (XPM) in bulk fiber mediums were discovered and pioneered by Dr. R. R. Alfano.
SC generation can be used to obtain multiple wavelength channels, as it can easily produce more than 100 optical longitudinal modes, while maintaining the coherency between the frequency modes. An advantage of using longitudinal (frequency) modes of the SC spectrum is that the resultant fixed channel spacing has the accuracy of a microwave oscillator. This means that entire wavelength channels can be fixed to grid frequencies by adjusting just one wavelength.
Other applications for SC-based sources and systems include WDM/TDM optical communication systems and other types of communications systems, optical frequency comb generators, high-resolution spectroscopy, optical metrology and optical tomography.
The SC arises from the propagation of intense pico-second (ps) or femto-second (fs) pulses through condensed matter, fibers, waveguides, or gaseous media (see, R. R. Alfano and S. L. Shapiro: Phys. Rev. Lett. 24, 584, (1970); Phys. Rev. Lett. 24, 592–594, (1970); Phys. Rev. Lett. 24, 1219, (1970) and U.S. Pat. No. 3,782,828, entitled “Picosecond Spectrometer Using Picosecond Continuum,” to Alfano et al.). Various processes are responsible for the generation of SC's, including self-, induced-, and cross-phase modulations and four-photon parametric processes, and soliton generation. When an intense laser pulse propagates through a medium, it changes the refractive index of the medium, which, in turn, changes the phase, amplitude, and frequency of the pulse. However, when two laser pulses having different wavelengths propagate simultaneously through a condensed medium, coupled interactions (i.e., cross-phase modulation) occur through the nonlinear susceptibility coefficients. These coupled interactions of two different wavelengths, can introduce phase modulation, amplitude modulation, and spectral broadening in each of the pulses due to the other pulse using cross-effects (see, U.S. Pat. No. 5,150,248, entitled “Terahertz Repetition Rate Optical Computing Systems, And Communication Systems And Logic Elements Using Cross-Phase Modulation Based Optical Processors,” to Alfano et al.). Dispersion plays a critical role in the SC, and in particular, about the zero group velocity dispersion region.
Currently, existing optical multiplexing systems such as wavelength/time division multiplexing (WDM/TDM) have limitations which are caused by the number of optical channels which can be provided by an optical channel source. Moreover, many optical channel generators require an individual laser for each optical channel which increases the complexity of the optical channel generator. Therefore, it is difficult to construct a WDM optical channel source that can provide more than 100 optical communication channels. For this reason, SC optical channel sources are a desirable means for providing multiple optical carriers (see, H. Takara. Optics & Photonics news, p. 48–51, March 2002). The advantages of using an SC source include its super-broadband spectrum to simultaneously generate more than 100 channels with fixed spectral channel spacing.
As a result of space-time self-focusing, multi-photon absorption and self-steeping, an “optical shock” wave forms inside the medium that gives rise to an even broader blue-shifted pedestal in the transmitted pulse spectrum.
There are several important applications of the SC pulse, such as a white-light probe pulse to study the fundamental temporal dynamics of elementary excitations in the fields of chemistry, biology and condensed matter. Additionally SC pulses can be used as a multi-wavelength optical source for optical fiber communication systems, and as an optical source for optical coherence tomography (OCT) to detect cells.
Shaping, signal processing, and time-space conversion of fs pulses can be achieved by linear and nonlinear manipulation of the spatially dispersed optical frequency spectrum within a grating pair and lens pulse shaper. This approach can be used for processing of information in ultra high-speed optical communications networks.
An ultrafast coherence cross-correlation technique can be used for the detection of coherence data streams as well as photon echo signals on an fs time scale.
Throughout this document, the term SC fiber is used to refer to SC fibers or other suitable SC producing mediums.
The high degree of spatial coherence of an SC source is highly desirable for use in wireless communication systems and for a frequency comb generation.